The present invention refers to a micromechanical switching device including at least two contact elements, which are provided at least partly movable relative each other and which via thermal actuation can close and open a circuit.
It is known to use either bimetal, electrostatic or magnetic force to displace a movable part to contact a fixed part. These are described closer in, e.g., Shifang Zhou et al, xe2x80x9cA Micro Variable Inductor Chip Using MEMS Relaysxe2x80x9d, Proc. TRANSDUCERS ""97, Chicago Jun. 16-19, 1997, pages 1137-1140 and Sumit Majunder et al, xe2x80x9cMeasurement and modelling of surface micromachined, electrostatically actuated microswitchesxe2x80x9d, Proc. TRANSDUCERS ""97, Chicago Jun. 16-19, 1997, pages 1145-1148 and William P. Taylor et al, xe2x80x9cIntegrated Magnetic Microrelays: Normally Open, Normally Closed, and Multi-polexe2x80x9d, Proc. TRANSDUCERS ""97, Chicago Jun. 16-19, 1997, pages 1149-1152. Also, a combination of bimetal and electrostatic force has been demonstrated (Shifang Zhou et al, xe2x80x9cA Micro Variable Inductor Chip Using MEMS Relaysxe2x80x9d, Proc. TRANSDUCERS ""97, Chicago Jun. 16-19, 1997, pages 1137-1140), where for the first time, the relatively large displacement, which can be achieved with bimetal was used to electrostatically increase the contact force afterwards. Common for above references is that they always are in an initial position without actuation, either ON or OFF, and to connect an actuation is required and as soon as it is interrupted the switching is reversed to its initial position.
A future application for micromechanical switches is in microwave circuits. Presently, electric switches are usually used, but these are, specially for high-frequency (HF) applications, afflicted with large losses; therefore in many applications it is desirable to exchange these for mechanical switches with low signal losses. Micromechanically produced switches can be integrated into different types of passive and active HF components such as waveguides, coils, capacitances and transistors. The known micromechanical switches, however, require actuation when activated and consequently consume energy when they are not in their initial position, this is a drawback specially when a battery is used for operation.
Through WO 95/34904 a micromechanical memory sensor including two arms is known, which mechanically switch when a threshold value of a variable condition is detected. The mechanical switching can be detected by means of a circuit for read out. Actually, this patent specification describes only a sensor. However, the possibility of using it as a switch is mentioned. No closer description of the switch and its function are expressed. Both at locking and unlocking, an arm is deflected more than the other one and snaps by when sufficient force is obtained. This procedure wears hard on the free ends of the arms and contact surfaces thereof. If a switch operates in this way, it should have very low length of life irrespective of being used to connect signals or not. Furthermore, the built-in tensions are not used in an advantageous way. The built-in tensions are used to calibrate the sensitivity of the sensors. Moreover, both arms according to this document are displaced in same direction when they are excited. Additionally, the manufacturing process according to this document is very complicated. It requires frequent back-etching and double-faced registration.
U.S. Pat. No. 3,761,855 discloses a switch including two thermally controlled arms. The switch is not based on micromechanical technics. The built-in tensions are not used at normal temperature, which implies that in the open position very poor insulation characteristics are obtained. This is very important in both electric (RF) and optical applications. Furthermore, the arms are displaced orthogonally relative to each other, which gives a complicated structure to manufacture, as the arms are arranged in different levels. The method used to interlock the free ends of the arms is complicated and clumsy. This invention is also sensitive for variations in ambient temperature, why temperature compensated arms have been introduced.
One object of the invention is to provide a bistable switch which requires low power consumption and/or has memory function at power failure.
Another object of the invention is to extend the technology of the micromechanical switches to also include bistable switches, which only consume energy at switching and maintain its state at power failure. Yet, one object of the switch according to the present invention is to enable larger insulation distance in an open position than the known types whereas it comprises two movable parts instead of one movable and one stationary. The invention benefits from the privilege of creating switch arms with high intrinsic stress, by using two arms with high oppositely directed intrinsic stress a high contact pressure and a self-locking function in the closed position is obtained. If a further increase of the contact pressure is required, the invention may also be arranged so that an electrostatic increase of the contact pressure can be used.
Yet, one object of the invention is to provide a micromechanical switch in which the excitation pulses are generated in such a way that connection and disconnection is achieved very leniently for the contactors.
Moreover, the invention has an objective to use the built-in tensions in an explicit way to obtain distinguished insulation distance between the contact elements when the switch is in an open position.
When manufacturing a micromechanical switch according to the invention, the order of the appearance of the material in the switching elements is inverted to achieve good insulation distance as the switching elements can be displaced in different directions.
The switching device, according to the invention, is cheap to produce and can be produced through a batch based thin and/or thick film technique.
The switch according to the invention is also considerably less sensitive to variations of the operation temperature through inherent much larger margins in both open and closed positions.
These objectives are obtained by the initially mentioned contact elements at least partly consisting of at least two materials with essentially different thermal expansion coefficients.